A team of US researchers have found how a coronavirus variant that had emerged in California interacts with the immune system and its potential to spread.

For our comprehensive coverage and latest updates on COVID-19 click here.

California's variant actually comes in two different forms, dubbed B1427 and B1429, and each has a unique combination of mutations. But they are paired as a single variant because they share a few distinct mutations that affect the spike protein. The variant first detected, in January 2021, is more transmissible than the unmutated strains of SARS-CoV-2, the virus that causes COVID-19, from which it evolved, preliminary data had suggested. Now, a multifaceted collaboration between researchers at the University of California-San Francisco, Gladstone Institutes, and other organisations across California found that California's homegrown variant can infect people who already had COVID-19.

"The take-home message is that everyone should get vaccinated, even if you've been infected before", said Charles Chiu, Director of the UCSF-Abbott Viral Diagnostics and Discovery Centre. For the study, published in the journal Cell, the researchers sequenced samples from 2,172 COVID-19 swab tests conducted in dozens of California counties. Analysis of the sequences suggested that the variant emerged in May 2020 and subsequently gave rise to the two current forms. Between September 1, 2020, and January 29, 2021, the prevalence of the variant in the sequenced samples rose from 0 per cent to more than 50 per cent of cases. Its transmissibility appeared to be as much as 24 per cent higher than unmutated viral strains, and people infected with the new variant had two times more virus present in their swab samples than people infected with unmutated strains.

The researchers generated "pseudoviruses" harmless viruses that contain SARS-CoV-2 spike proteins with different mutations. The team compared three different spike protein mutations: two that are found in the California variant, and one that is commonly found in predominant variants worldwide. They found that pseudoviruses with one of the California variant's mutations, known as L452R, was better able to infect human cells than pseudoviruses carrying other mutations. "This suggests that the L452R mutation enhances the ability of the virus to enter a cell, which could explain why the California variant has increased transmissibility," said Melanie Ott, Director of the Gladstone Institute of Virology.

Further, the team tested the variant against blood samples from people who had received both doses of either the Pfizer or Moderna vaccine, as well as from people who had previously been infected with what was most likely an unmutated form of SARS-CoV-2. The experiments showed that the new variant was only moderately neutralised by antibodies from previously infected patients. But antibodies from vaccinated patients showed a higher degree of neutralisation.